In the radio communication, as a technique of using a plurality of transmission and reception antennas (multi-antenna), there have been studied a technique of transmission by a multiplicity of antennas and reception by a multiplicity of antennas (MIMO: Multi Input Multi Output) in which different streams are transmitted from each antenna, thereby improving throughput, and a beamforming (which may be represented as BF hereinafter in an appropriate time) technique in which multiplex transmission is performed by weighting an identical stream, thereby improving reception quality (Non-patent document 1).
Further, in Patent document 1, in a mobile communication system having a plurality of antennas, in order to improve spatial efficiency, there has been disclosed a method of adaptively performing switching control in a mixed environment of space multiplexing (SDM: Space Division Multiplex, SDMA: Space Division Multiplex Access), according to a traffic condition in a communication area.
However, conventionally, the MIMO multiplexing and beamforming have been recognized as independent techniques because of the passage of different propagation paths, and a study taking mutual fusion into consideration has hardly been made.
Namely, in the beamforming mode, because transmission weighting is made, a passing propagation path is different from that in the MIMO multiplexing, and accordingly, it is difficult to simultaneously control both methods using a single common pilot. Therefore, the necessity of separate pilots has been studied. As a result, in any of the above-mentioned documents, there is neither disclosure nor suggestion about a concrete structure of controlling switching between the MIMO (SDM is also included in MIMO) and the beamforming.    [Non-patent document 1] “Spectral Efficiency of Wireless Systems with Multiple Transmit and Receive Antennas”, F. R. Farrokhi et al., PIMRC2000 vol. 1 page 373-377.    [Patent document 1] the official gazette of the Japanese Unexamined Patent Publication No. 2004-201296.